Polished unpainted stainless steel extinguishers (water/foam/powder) & polished unpainted aluminium extinguishers (CO2) are still manufactured, sold and are fully legal.
The only legal requirement (for post 2002 extinguishers) is that must be CE marked by an approved body and meet the Pressure Equipment Directive. This applies to all models including aerosol extinguishers & those in sizes not included in EN3
EN3 is a manufacturing standard, not an installation standard (that's BS 5306-8, currently under revision) and isn't obligatory, hence the continued manufacture of 1 litre foams for the public service transport industry, small 600g & 900g extinguishers for the leisure/domestic market (and some workplaces)and of course unpainted models.
Pure EN3 didn't even permit the colour coding we use, we managed to get a national specification allowance to adopt 5% (& later 10%) colour coding.
The key element for correct use on the correct fire is not in the red body or the colour coding, but the pictograms, which unpainted extinguishers still use....& of course staff training
Unpainted extinguishers are usually made to the construction & performance requirements of BS EN 3, but cannot be kitemarked if polished bare metal as they are not red, which is a requirement of the standard. The same applies to "Stainless steel effect" CO2 extinguishers (actually polished aluminium cylinders), so there are no safety or efficacy issues.
There is no obligation for an extinguisher to be BSI kitemarked (some use the Lloyds Register or LPCB) even if it is red. The Fire Safety Order just requires 'suitable & sufficient' fire fighting equipment - in theory a bucket of water labelled 'FIRE' could be 'suitable & sufficient' and that certainly isn't to BSEN3!
As long as staff are suitably aware in training that not all extinguishers are red and the appropriate signage is in place there is no reason not to use them and this can be documented in the FRA. Best practice says you shouldn't mix different colours (paint/not paint) at fire points, but it's not unreasonable for public areas to be all unpainted, back of house all red for example.
After all for over three decades we had extinguishers that were unpainted, red, all black, all cream, all blue and all green and nobody didn't know what they were.